There has been conventionally known a scroll expander as disclosed in Patent Document 1. The scroll expander disclosed in Patent Document 1 is integrally provided with a high pressure portion into which a high-pressure working fluid (refrigerant) is introduced, a drive portion driven by expansion of the working fluid supplied from the high pressure portion; a low pressure portion allowing the working fluid to flow out of the scroll expander after being expanded to a lower pressure in the drive portion; a communication passage (bypass passage) through which the high-pressure portion communicates with the low-pressure portion while bypassing the drive portion; and a valve mechanism (bypass valve) configured to open and close the communication passage.